A typical data network comprises a number of source nodes, each source node receiving traffic from numerous traffic sources, and a number of sink nodes, each sink node transmitting data to numerous traffic sinks. The source nodes can be connected to the sink nodes directly or through core nodes. Source nodes and sink nodes are usually paired so that a source node and an associated sink node are included within a single edge node.
The capacity of a given data network may be defined by the capacities of the edge nodes and the core nodes. Each link between nodes (e.g., a link from a source node to a core node or a link from a core node to a sink node) may comprise multiple channels. Where an optical link is divided into channels using Wavelength Division Multiplexing (WDM), each channel in the optical link is defined by a separate modulated wavelength. Additionally, the link, or channels of the link, may be sub-divided in time using Time Division Multiplexing (TDM). In a TDM scheme, a set number of time slots makes up a TDM frame. A core node that acts to connect source nodes to sink nodes using multi-channel links may switch an entire incoming link to an outgoing link (link switching), an incoming channel to an outgoing channel (channel switching) or an incoming sub-divided channel to an outgoing sub-divided channel (time-sharing switching). In time-sharing switching, a received optical signal may be switched to a first sink node during one time interval and switched to a second sink node during an immediately subsequent time interval. Accordingly, a switch must be capable of changing the destination of a received signal in a very short period of time. Switches capable of such switching are said to be capable of fast-switching.
Fast-switching, high-capacity, optical switches are needed to realize an agile optical-core network, that is, an optical-core network that may adjust swiftly to changes in desired connectivity between edge nodes. A core node comprises one or more switches. The degree of a switch (or a core node) is a measure of a number of input ports and output ports. To construct a high degree, fast-switching, optical core node using optical-switch modules of smaller sizes, it is known to use an aggregation of the optical-switch modules in a multi-stage arrangement. Where a typical space switch need only be configured to connect an input port to an output port, a core node comprising several optical-switch modules may need to be configured to connect an input port in a first switch to an output port of a second switch. Such a need may be met by establishing a “path” across the core node from the first stage to the second stage. If a direct connection from the first stage to the second stage is unavailable, such a path may traverse one or more intermediate stages.
The construction of a high-capacity, multi-stage, optical core node using optical-switch modules would be considered easily manageable if high path-establishment speed was not a requirement. For instance, high path-establishment speed is not a requirement in the case of conventional cross connectors. However, a relatively long delay required to change a path across an optical core node may preclude the use of such an optical core node for time-sharing switching schemes such as TDM switching or burst switching. In the absence of such time-sharing switching schemes, the multi-stage optical core node becomes a channel-switching cross-connector and a network based on such a core node may be forced to perform such measures as multiple edge-to-edge hops and/or intra-core hops to inter-connect certain edge nodes. These measures may significantly increase the complexity, and degrade the performance, of a network. Ideally, every source node in a given network should be able to reach every sink node in the given network either directly or via a single core node. However, when none of the core nodes to which a source node connects subsequently connects to a desired sink node, there may be a necessity to send a data stream to an intermediate edge node that connects to a core node that does connect to the desired sink node. Such use of an intermediate node may be called “tandem switching”.
The number of sink nodes that a given source node can reach without switching at an intermediate edge node is referenced herein as the reach index of the source node. Slow switching, in contrast to the fast switching mentioned above, may limit the reach of a source node and may necessitate tandem switching for data streams of low traffic intensity. This is due to the coarse granularity resulting from slow switching, which increases the number of hops. A data stream is defined as data that is transferred from a source node to a sink node over a particular path (e.g., via a particular core node). Tandem switching may be required when channel switching is used in the core because the number of channels emanating from a source node would typically be smaller than the number of sink nodes addressable by the source node.
Hereinafter, a link from a source node to a core node is called an uplink and a link from a core node to a sink node is called a downlink. A channel in an uplink is called an upstream channel and a channel in a downlink is called a downstream channel. Data carried by an uplink or an upstream channel is identified as upstream data and data carried by a downlink or a downstream channel is identified as downstream data
An electronic edge node can realize a high reach index due to the inherent fast-switching ability of electronic edge nodes, which enables capacity division into small units. As stated above, the reach index of a reference edge node is the number of other edge nodes that can be reached by the reference edge node, directly or through core nodes. An optical core node, however, is preferably bufferless and, hence, requires precise time coordination with the source nodes in order to create paths of fine granularities through time sharing. To summarize, a fast-switching core node may independently route individual data blocks defined using time-sharing switching techniques such as TDM switching or burst switching. Realizing a high-capacity network requires high-capacity, fast-switching core nodes. Preferably, the core nodes include optical switches.
McGuire (U.S. Pat. No. 5,889,600, issued Mar. 30, 1999) discloses a modular switch operated in a channel switching mode comprising a plurality of star couplers, connecting to a plurality of input WDM links and a plurality of output WDM links. Each WDM link comprises a number of wavelength channels equal to the number of star couplers. Each input WDM link is demultiplexed into its constituent wavelength channels and each of the individual wavelength channels connects to an input port of one of the star couplers. Wavelength converters are provided at the output ports of the star couplers. Each output WDM link carries multiplexed optical signals received from an output port of each star coupler. The modular switch allows a wavelength channel from any input port to connect to any of wavelength channel in a subset of the output ports of the star couplers. For example, using 32×32 star couplers, 32 WDM input links and 32 WDM output links, each input link and each output link carrying 32 wavelength channels, a specific wavelength channel in an input link can be switched to any one of a subset of 32 of the 1,024 output ports of the 32 star couplers.
Multi-stage, optical switch structures that switch channels are known in the prior art. For example, Kuroyanagi (U.S. Pat. No. 6,154,583, issued Nov. 28, 2000) describes an optical switch configured as a multi-stage circuit, with each of the stages including a plurality of space switches. An arrangement of optical amplifiers is also described. Such structures, however, are limited to channel switching granularity, which may be considered too coarse for future applications.
Bala et al. (U.S. Pat. No. 6,335,992, issued Jan. 1, 2002) describe a scalable multi-stage optical cross-connect. The multi-stage optical cross connect comprises a plurality of first stage switch matrices, a plurality of middle stage switch matrices having input ports and output ports, and a plurality of last stage switch matrices having input ports and output ports. Each of the first stage switch matrices has a plurality of input ports, each input port receiving an input communication signal, and a larger number of output ports, where the first stage switch matrices switch the input communication signals to selected output ports. The input ports of the middle stage switch matrices are coupled to the output ports of the first stage switch matrices for receiving communication signals output from the first stage switch matrices. The middle stage switch matrices switch communications signals received at their input ports to their output ports. The input ports of the last stage switch matrices are coupled to the output ports of the middle stage switch matrices for receiving communication signals output from the middle stage switch matrices. The last stage switch matrices switch communications signals received at their input ports to their output ports. In addition, the middle stage itself can be recursively a multistage switch.
Neither of the above two disclosures suggests the use of a time-sharing scheme, such as TDM, in a bufferless multi-stage switching node. A node structure that permits scalability and can employ time-sharing techniques is required, and methods of circumventing the difficulty of scheduling signal transfer in bufferless, multi-stage, time-sharing, optical switching nodes are required to enable the realization of such nodes and, ultimately, an efficient network that scales to capacities of the order of several petabits/second.